In recent years, in terms of global environmental protection, cases where heat-pump air-conditioning apparatuses which use air as a heat source are introduced even in cold regions, instead of known boiler-type heating devices which burn fossil fuels to perform heating, have increased. With heat-pump air-conditioning apparatuses, heating is able to be performed more efficiently by the amount of heat supplied from air, in addition to electric input to a compressor. In contrast, however, when the temperature of outside air is low, frost is deposited on an outdoor heat exchanger serving as an evaporator, it is therefore necessary to perform defrosting to melt frost deposited on the outdoor heat exchanger. As a method for performing defrosting, there is a method for reversing a refrigeration cycle. With this method, however, heating of a room is stopped during defrosting. Therefore, there is a problem of impairing comfort.
As a method for being able to perform heating even during defrosting, methods have been developed (see, for example, Patent Literature 1, Patent Literature 2, and Patent Literature 3), in which an outdoor heat exchanger is divided, with part of the outdoor heat exchangers performing defrosting, while the other heat exchangers being caused to operate as evaporators, which receive heat from air and thus perform heating.
In Patent Literature 1, in the case where an outdoor heat exchanger is divided into plural parallel heat exchangers and defrosting of one parallel heat exchanger is performed, by closing a flow rate control device which is installed near the other parallel heat exchanger and opening a flow rate control device at a bypass pipe which causes a refrigerant to take a detour to the inlet of the parallel heat exchanger from a discharge pipe of a compressor, part of a high-temperature refrigerant that has been discharged from the compressor is caused to flow into the parallel heat exchanger directly. Then, after defrosting of the one parallel heat exchanger is completed, defrosting of the other parallel heat exchanger is performed. At this time, defrosting of the other parallel heat exchanger is performed in a state where the pressure of a refrigerant inside the other parallel heat exchanger is substantially the same as the suction pressure of the compressor (low-pressure defrosting).
In Patent Literature 2, plural outdoor units and at least one or more indoor units are provided. The direction of connection of a four-way valve in only an outdoor unit that includes an outdoor heat exchanger subjected to defrosting is reversed relative to that in a heating operation, so that a refrigerant that has been discharged from a compressor is caused to flow into the outdoor heat exchanger directly. At this time, defrosting is performed in a state where the pressure of a refrigerant in the heat exchanger subjected to defrosting is substantially the same as the discharge pressure (high-pressure defrosting).
In Patent Literature 3, an outdoor heat exchanger is divided into plural parallel heat exchangers. By causing part of a high-temperature refrigerant that has been discharged from a compressor to flow into the parallel heat exchangers alternately, and defrosting of the parallel heat exchangers is performed alternately. Accordingly, continuous heating can be performed without reversing a refrigeration cycle. Further, in Patent Literature 3, medium-pressure defrosting is proposed in which defrosting is performed in a state where the pressure of a refrigerant in a parallel heat exchanger subjected to defrosting is not the same as the discharge pressure or the suction pressure but is slightly higher than that at 0 degrees Centigrade when converted into a saturation temperature and the refrigerant is returned to an injection part of an injection compressor.